As is well known, electromagnetic flow sensors can measure the volumetric flow rate of an electrically conductive fluid flowing through a measuring tube of the flow sensor. A magnetic-circuit arrangement coupled to excitation electronics produces a magnetic field of maximum density which passes through the fluid within a measurement volume in sections, particularly in the area with high flow velocity, at right angles to the direction of fluid flow, and which closes essentially outside the fluid. The measuring tube is therefore made of nonferromagnetic material, so that the magnetic field will not be adversely affected during measurements.
Due to the movement of the charge carriers of the fluid in the magnetic field, according to the magnetohydrodynamic principle an electric field of a given strength is produced at right angles to the magnetic field and to the direction of fluid flow. By two electrodes spaced in the direction of the electric field and by evaluation electronics connected to these electrodes, a voltage induced in the fluid can thus be measured. This voltage is a measure of the volumetric flow rate. To pick off the induced voltage, use is made of either galvanic electrodes which are in contact with the fluid, or capacitive electrodes, which do not contact the fluid.
The flow sensor is so designed that the induced electric field closes outside the fluid practically only via the evaluation electronics connected to the electrodes. To guide and effectively couple the magnetic field into the measurement volume, the magnetic-circuit arrangement commonly comprises two coil cores which are disposed at a distance from each other, particularly diametrically opposite each other, along a circumference of the measuring tube, and have respective free end faces located opposite each other, particularly mirror-symmetrically with respect to each other.
By means of a coil assembly connected to the excitation electronics, the magnetic field is coupled into the coil cores in such a way as to pass through the fluid flowing between the two end faces, at least in sections, at right angles to the direction of flow.
Because of the high mechanical stability required for such measuring tubes, the latter preferably consist of an external support tube of a predeterminable strength and width, particularly of a metallic support tube, whose inner surface is covered with an insulating material of predeterminable thickness, the so-called liner.
U.S. Pat. No. 3,213,685 discloses an electromagnetic flow sensor comprising:                a measuring tube having an inlet-side first end and an outlet-side second end which can be inserted into a pipe in a pressure-tight manner and comprises:                    a nonferromagnetic support tube as an outer covering of the measuring tube,            a tubular liner located in a lumen of the support tube and made of an insulating material for conducting a flowing fluid isolated from the support tube, and            a reinforcing body embedded in the liner for stabilizing the latter;                        a magnetic-circuit arrangement disposed at the measuring tube for producing and guiding a magnetic field which induces an electric field in the flowing fluid; and        a first electrode and a second electrode for picking up a voltage from the electric field.        
The liner serves to chemically isolate the support tube from the fluid. In the case of support tubes of high electric conductivity, particularly in the case of metallic support tubes, the liner also serves to provide electric isolation between the support tube and the fluid in order to prevent the electric field from being short-circuited via the support tube.
Thus, by a suitable design of the support tube, the strength of the measuring tube can be adapted to the mechanical stresses exerted in the respective application, while by the liner, the measuring tube can be adapted to meet the chemical, and particularly hygienic, requirements in force for the respective application.
The liner, which is formed of plastic, is commonly made with an open-pore reinforcing body completely embedded therein, particularly a metallic reinforcing body. This reinforcing body serves to stabilize the liner mechanically, particularly against pressure changes and thermally induced variations of volume. JP-Y 53-51 181, for example, shows a tubular reinforcing body whose wall is provided with holes for receiving the liner material. This reinforcing body is located in and is coaxial with a support tube, and is completely surrounded by insulating material.
To optimize the density of the magnetic field and thus improve the sensitivity of the flow sensor, the end faces of the coil cores are designed as pole pieces with as large an area as possible and a given curvature. By shaping this curvature in a suitable manner, the density of the magnetic field in the measurement volume can be selectively optimized. This also optimizes the three-dimensional shape of the electric field and, thus, the dependence of the voltage induced in the fluid on the flow velocity of the fluid.
The three-dimensional shape of the magnetic field in the fluid and, thus, the accuracy of the flow sensor, besides depending on the form of the two end faces, are also determined by the distance between the two end faces. The farther the two end faces are apart, the weaker the electric field and the higher the sensitivity of the measured voltage to disturbances, such as changes in flow behavior or temperature variations in the fluid.
Therefore, to improve the accuracy of the flow sensor, on the one hand, the end faces should be spaced a minimum distance apart and, on the other hand, their curvature should be adapted to the respective optimum curvature as accurately as possible. In commercially available flow sensors, therefore, the pole pieces are shaped essentially according to the outer contour of the tube and are so disposed on the measuring tube that their end faces rest directly on the liner; see, for example, U.S. Pat. No. 4,825,703.
U.S. Pat. No. 5,664,315 discloses a method of manufacturing a measuring tube of an electromagnetic flow sensor whose inner surface is provided with a liner. Prior to the introduction of the liner into the support tube, an expanded-metal lattice which mechanically stabilizes the liner is fitted as a prefabricated reinforcing body. The liner is introduced by filling a liquefied insulating material into the measuring tube and allowing it to solidify. After having solidified, the insulating material surrounds the reinforcing body and thus forms the liner. The liner is preferably formed using injection-molding or transfer-molding techniques.
It is also common practice to install a completely prefabricated liner in the support tube. JP-A 59-137 822, for example, shows a method in which the liner is formed by softening an external plastic film and an internal plastic film surrounding a tubular, porous reinforcing body of high-grade steel.